Calculate Voltage Drop in Low Voltage Landscape Lighting

Calculate Voltage Drop Landscape Lighting

Obviously, low voltage lighting has lots of advantages. One of them not being an abundance amount of electricity to power your landscape lights.

With low voltage lighting, you are going to have a drop off of power the longer the cable is, and the more lights you have on the fixture. This could make some lights dim more than others, especially the ones farther down the line, or further from the transformer.

Sounds like common sense right? Well, here a good formula to help calculate how much power you will lose. Knowing this will help you pick the right transformer in the first place, or at least know in advance that you will need more than one transformer. Basically allowing you to do the job right the first time.

The Formula is:

0.0011 x Total watts on line x Cable length (in feet) = Voltage Drop %


So lets say you have ten, 9 watt lights on a 40 ft line.

Your calculation would be .0011 x 90 (watts) x 40 (ft) = 3.96% voltage drop.

So, your voltage drop would be 3.96%

Dimming will usually show up if you are losing 7-8% or higher per light.

So, to continue the example here, you have these 10 lights, which are going to use about 90 watts. If you wanted to make sure you had more than enough power, add in the voltage drop, which was 3.96%.

So 90 (watts) x .0396 = additional 3.56 (watts)

Account for the Drop when Picking the Right Transformer

So, to account for the voltage drop, add the 3.56 back to the original 90 watts you were planning for (90) + 3.56), and you have 93.56 watts.

Use this number when adding up your total wattage to pick you transformer. This will keep you from having to do the whole job over, or from buying an additional transformer.